This invention relates to the treatment of hydrogen sulfide wastes to produce hydrogen in addition to sulfur and more particularly to a process characterized by energy savings, decreased levels of impurities and other benefits.
Significant quantities of hydrogen sulfide as a waste stream are produced in the refining industry where "sour" hydrocarbon feedstocks are upgraded by treatment with hydrogen usually obtained from a reforming process. The overall operation results in the consumption of large amounts of hydrogen while producing similar levels of hydrogen sulfide. A similar situation exists in the natural gas production industry where H.sub.2 S must be removed from the crude gas and then disposed of. Furthermore, hydrogen sulfide produced in the metals refining industry could also be treated by this invention.
The current treatment technology for hydrogen sulfide waste is based on the Claus chemistry which depends on the mutual reduction/oxidation between sulfur dioxide and hydrogen sulfide to produce water and elemental sulfur.
However, the Claus chemistry requires a second stage of the waste-treatment process commonly identified as the "tail-gas clean-up" process (e.g., SCOT or Beavon) to convert sulfur-containing impurities (such as COS, CS.sub.2, SO.sub.2 and sulfur aerosol) back into hydrogen sulfide for recycle to the Claus unit. This "tail-gas clean-up" process requires an additional reducing reagent and additional energy requirements. Unconverted impurities are burned to form SO.sub.2 which is then emitted to the environment.
Industry has investigated several alternatives to the Claus chemistry in order to recover the hydrogen used for the sweetening process but none have reached the commercial stage. These alternatives have included direct thermal decomposition, chemically promoted thermal decomposition, electrolysis, a combination of electrolysis and chemical decomposition, and liquid metal conversion. While each of the alternatives produced hydrogen instead of water, they were abandoned because of poor yields, high energy costs or process limitations. Accordingly, a major interest remains to develop a combined hydrogen and sulfur recovery process for hydrogen sulfide waste treating.
Recently, a technology has been introduced at the development stage to treat hydrogen sulfide waste under plasma conditions which dissociate hydrogen sulfide into hydrogen and sulfur. A general description may be found in "Hydrogen Sulfide Waste Treatment by Microwave Plasma Dissociation" by John B. L. Harkness, Anthony J. Gorski and Edward J. Daniels, Presented at the 25th Intersociety Energy Conversion Engineering Conference on August 12-17, 1990, Reno Nevada, and in the Soviet literature as represented by "Dissociation of Hydrogen Sulfide in a Plasma" by A. V. Balebanov, et al., Doklady Physical Chemistry, Proceedings of Academy of Sciences of the USSR, Translated from Doklady Akademii Nauk SSSR, 282(3):675-660 (July 1985) and "Effect of Spatial Nonequilibrium in the Dissociation of Hydrogen Sulfide in a Nonuniform Plasma" by V. D. Rusanov et al., Soviet Physical Doklady 30(7); 592-594 (July 1985). FIG. 1 represents a general flow diagram of the process. As illustrated, a sour gas, such as would be produced by an amine purification unit, is fed to a plasma reactor where a plasma is generated with microwave or radio-frequency energy. Cooling water is fed to the power supply to limit the temperature. The product stream from the plasma unit includes hydrogen, sulfur and various impurities including COS, CS.sub.2, SO.sub.2 and CO as well as CO.sub.2 and H.sub.2 O contained in the original sour gas. The product sulfur is condensed and sulfur aerosols are removed in the sulfur scrubber by action of a liquid sulfur spray with the sulfur less a portion for recycle being removed.
Steam is also produced as illustrated. Hydrogen, unconverted H.sub.2 S, CO.sub.2, H.sub.2 O, and the impurities are removed as an overhead stream from the scrubber and fed to a compressor which provides sufficient pressure to return this stream to an existing purification unit (prior to the plasma reactor) as represented by an amine purification process.
As indicated, the new technology is primarily in the developmental stage although providing several advantages. Hydrogen is generated in the process which may limit the need for a reformer to supply hydrogen for the hydrodesulfurizer operation. Energy requirements are also lowered and environmental emissions are decreased. In a natural gas production facility, the hydrogen produced could be used as an auxiliary fuel or sold. However, further improvements in the process are needed.
One object of this invention is one or more improvements in a process to generate hydrogen and elemental sulfur from hydrogen sulfide. Other objects will be apparent from the following description.